1) Field of the Invention
The present invention relates to a differential M phase-shift modulator (M: M=2n), and particularly to a differential M phase-shift modulator suitable in use for a transmitter in an optical transmission system.
2) Description of the Related Art
In recent years, demand has increasingly been growing for installation of a next-generation 40-Gbit/s optical transmission system, and also a transmission distance and frequency utilization efficiency similar to those of a 10-Gbit/s optical transmission system have been demanded. As an implementation means thereof, research and development of the DPSK (Differential Phase Shift Keying) modulation method, which is superior to the NRZ (Non return to Zero) modulation method conventionally applied to systems of 10 Gbit/s or below in optical signal to noise ratio (OSNR) strength and nonlinear strength, has been boosted. Further, in addition to the aforementioned modulation method, research and development of phase modulation methods such as the DQPSK (Differential Quadrature Phase-Shift Keying) modulation featuring narrow spectra (high frequency utilization efficiency) has also been boosted.
Particularly, the DQPSK modulation method is a method in which two phase-modulated digital signals are transmitted simultaneously using signal light of one frequency. This method requires only half pulse repetition frequencies (for example, 20 GHz) with respect to the speed of data to be transmitted (for example, 40 Gbit/s) and the spectral bandwidth will be half that of the conventional NRZ modulation method, and therefore is superior in frequency utilization efficiency, wavelength dispersion strength, and device transmission properties. Thus, in a field of optical transmission systems, particularly optical transmission systems whose data speed exceeds 40 Gbit/s, applications of this modulation method are actively explored.
FIG. 15 is a diagram showing a configuration example of a Mach-Zehnder (hereinafter simply called the MZ type) DQPSK modulator that adopts a conventional DQPSK modulation method. A similar DQPSK modulator is also described in Patent Document 1 shown below. A DQPSK modulator 100 shown in FIG. 15 is comprised of two child MZ type light modulators 101 and 102, a parent MZ type waveguide 103 comprised of the child MZ type light modulators 101 and 102, and a π/2 phase shifter 104.
That is, each of the child MZ type light modulators 101 and 102 comprises MZ type waveguides 101a and 102a, modulating electrodes 101b and 102b, and bias electrodes 101c and 102c adapted to supply a bias voltage to stabilize an operating point. Then, light output from an LD (Laser Diode) 105 is input into the child MZ type light modulators 101 and 102. Data signals (DATA1, DATA2) output from a DQPSK signal source 106, on the other hand, are amplified by drivers 107 and 108 of differential output and then input into the modulating electrodes 101b and 102b of the child MZ type light modulators 101 and 102 respectively. These data signals are thereby to be modulated in the child HZ type light modulators 101 and 102 by the data signals (DATA1, DATA2) from the DQPSK signal source 106 respectively.
The π/2 phase shifter 104 performs a phase shift of at least one of signals modulated by the child MZ type light modulators 101 and 102 so that the signals have a phase difference of π/2 with each other, and the parent MZ type waveguide 103 multiplexes signal lights from the child MZ type light modulators 101 and 102 whose phase shift has been performed by the π/2 phase shifter 104 so that the light can be output as a DQPSK modulated light.
In the child MZ type light modulator 101, for example, continuous light from the LD 105 (See A in FIG. 16) is modulated by one system of coding data (DATA1) from the DQPSK signal source 106 to output an optical signal carrying information in two optical phases (0 rad or π rad) (See B in FIG. 16). Further, in the child MZ type light modulator 102, continuous light from the LD 105 (See A in FIG. 16) is modulated by another system of coding data (DATA2) from the DQPSK signal source 106 and the phase of the modulated light is shifted by φ=π/2 by the π/2 phase shifter 104. An optical signal carrying information in two optical phases (π/2 rad or 3π/2 rad) is thereby output (See C in FIG. 16).
Then, the modulated lights from the child MZ type light modulators 101 and 102 are multiplexed by a multiplexing waveguide constituting the parent MZ type waveguide 103 before being output. That is, by multiplexing the modulated lights from the child MZ type light modulators 101 and 102, the parent MZ type waveguide 103 can output an optical signal carrying information in four optical phases (π/4, 3π/4, 5π/4, and 7π/4) with constant light intensity, that is, a DQPSK-modulated optical signal.
Meanwhile, Patent Document 1 describes a DQPSK modulator in which pilot frequency components are provided to a voltage signal supplied to the π/2 phase shifter 104 and the child MZ type light modulators 101 and 102 to monitor the pilot frequency components in output from the multiplexing waveguide constituting the parent MZ type waveguide 103 to perform feedback control.
[Patent Document 1] PCT International Publication No. WO 03/049333
In the DQPSK modulator shown in FIG. 15 or that described in Patent Document 1, it is necessary to make an electrode length of the modulating electrode 101b and that of the π/2 phase shifter 104 shorter to achieve lower losses and reduction in device size. Thus, in order to create a phase difference of π/2 between an optical signal output from the child MZ type light modulator 101 and that output from the child MS type light modulator 102 when viewed from the parent MZ type waveguide 103, there are problems that a required supply DC (Direct Current) bias value becomes larger and a supply part of a DC bias to the π/2 phase shifter 104 and a feedback control circuit for controlling the DC bias become large in size. There is also a problem that, in addition to components such as the child MZ type light modulators 101 and 102, the π/2 phase shifter 104 is needed, and therefore the chip configuration and module configuration of the modulator become complicated.